Silicon is the main semiconductor material used to fabricate today's commercial solar cells. The majority of commercial solar cells are fabricated from a monocrystalline or polycrystalline silicon wafer. A p-n junction is formed in the silicon wafer by, for example, diffusing n-type atoms in a p-type silicon wafer.
A large amount of solar cells are fabricated using a boron doped wafer (p-type). The boron atoms introduced in the silicon material tend to form boron-oxygen (B—O) complexes. These complexes can form electrically active defects when exposed to radiation, such as visible light. Similarly polycrystalline wafers are known to contain a wide variety of metal impurities which may subsequently change form and introduce recombination centers. Electrically active defects throughout a solar cell affect the lifetime of charge carriers causing reduced performance.
Techniques have been used in the art to reduce the effect of defects in p-type silicon solar cells. However these techniques only apply to defects that are already present in the silicon. Thus any defects that form subsequent to this processing will still adversely impact the solar cell efficiency. P-type Czochralski (Cz) silicon solar cells, for example, are known to suffer from light induced degradation (LID) which may causes a drop in efficiency of up to 3% absolute during the first 24-48 hours of operation. Similarly polycrystalline silicon solar cells have been shown to suffer from a similar light induced degradation in performance, although on a much slower timescale compared to Cz wafers.
Under typical solar cell processing conditions many of these defects have not yet formed. Thus in general these techniques require hours or even days to stabilise the silicon material. This is generally incompatible with the high throughput solar cell manufacturing environment.
For example U.S. Pat. No. 8,263,176 describes techniques to stabilise the performance of solar cells using high temperature. According to U.S. Pat. No. 8,263,176 a light source can approximately accelerate the stabilising treatment process up to a factor of 8. However, under illumination stronger than 1000 W/m2, the acceleration decreases and reaches saturation. The resulting stabilisation process is performed on a hot plate at a temperature of 160° C. The solar cell is illuminated at the same time to generate excess minority carriers. The solar cell is held in this state for about 30 minutes before undertaking further processing, e.g. wiring and encapsulation in modules.
There is a need in the art for a process and equipment capable of stabilising the silicon material by forming the electrically active defects more rapidly.